In general, an automotive air-conditioning system serves to maintain the interior temperature of an automobile below an outside temperature through a cyclic operation of compression, condensation, expansion and evaporation of refrigerant. The above cyclic operation generally requires a compressor, a condenser, an expansion valve and an evaporator. The compressor is typically powered by an accessory drive. The compressor may be a fixed displacement or a variable displacement type compressor. The compressor operation may be controlled by an engagement or disengagement of a clutch, depending on the conditions of the system.
The clutch is typically located between the accessory drive and the compressor. The clutch typically incorporates a pulley that engages an accessory drive belt. The type of clutch most often used in automotive air conditioner compressor applications is an electromagnetic clutch, although mechanical, hydraulic or other clutch types could also be used. An electromagnetic clutch is usually preferred due to ease of packaging and control. An electromagnetic clutch typically operates by using an electromagnet to hold an armature to a rotor. In many automotive air conditioners, the armature is attached to the compressor and the rotor is attached to the accessory drive. Some compressors engage and disengage the clutch to cycle the automotive air conditioner compressor on and off as needed.
During the operation of the compressor, an over-torque condition may occur when the normal operating torque of the compressor is exceeded due to a mechanical problem in the compressor, such as a bearing seizing. Some automotive air conditioner compressors that are equipped with clutches are also configured to stop coupling torque between an accessory drive and the compressor if the compressor causes an over torque condition on the accessory drive. This is desirable to avoid damage to the accessory drive that may render other components connected to the accessory drive, such as an alternator or coolant pump, inoperative.
When an over-torque condition occurs, the torque applied to the clutch may exceed the ability of the magnetic field from the rotor to hold the armature in place. The torque from the accessory drive may cause the rotor to slip and rotate relative to the armature. Heat may be generated by the friction between the armature and rotor. If the clutch is not disengaged, heat generated by friction could damage the accessory drive belt or the clutch bearings. There are several ways an electromagnetic clutch can be disengaged to decouple the compressor from the accessory drive when an over-torque condition occurs. Excessive heat generated by friction between the rotor and armature may cause a thermal fuse to open and de-energize the coil 24. Alternately, a thermal sensor may detect excessive heat and trigger an electronic compressor controller to de-energize the coil 24. Optionally, a motion sensor can detect that the compressor has stopped rotating or that a speed differential exists between the compressor and accessory drive and provide an indication for an electronic compressor controller to de-energize the coil 24.
The methods previously used to disengage the clutch under over-torque conditions have potential disadvantages. For example, temperature tolerances in thermal fuses may cause a thermal fuse to open at temperatures within normal operating conditions or the thermal fuse may fail to open during over-torque conditions. A thermal fuse may not quickly disengage the clutch to decouple the compressor from the accessory drive because of the time needed for friction to generate excessive heat. The typical time required for a thermal fuse to open for an over-torque condition event may be 50 to 80 seconds. If the accessory drive pulley 18 has a low engagement percentage, the accessory drive pulley may experience belt slip during an over-torque condition before slip between the armature and rotor occurs. In these cases, the accessory drive belt may be damaged before the thermal fuse opens. Therefore, a thermal fuse is not preferred for these applications.
Likewise, a thermal sensor may not quickly disengage the clutch to decouple the compressor from the accessory drive because of the time needed for friction to generate excessive heat. A motion detector may not quickly disengage the clutch to decouple the compressor from the accessory drive due to the time required to detect that the compressor has stopped rotating or that a speed differential exists between the compressor and accessory drive. Additionally, thermal sensors and motion sensors are more expensive than thermal fuses and additionally require electrical connections to an electronic controller, and so are also not preferred.